1. Field of the Invention
The present invention relates to the selective production of C6-C12 hydrocarbons useful as automotive fuel components. In particular, the present invention concerns a process for dimerizing lower, olefinic hydrocarbons in the presence of a catalyst under process conditions allowing for selective dimerization.
2. Description of Related Art
Light olefin dimers are useful intermediates in the manufacture of different products, like alcohols, ketones and carboxylic acids. Highly branched trimethylolefins and trimethylparaffins are useful as gasoline octane number enhancers.
Many processes for utilization of light olefins for the production of high quality transportation fuels are known. The Mobil Olefin to Gasoline and Distillate (MOGD) process converts propylene and butylene to olefinic distillate in high yields. The MOG or Mobil Olefins to Gasoline process is an extension of the MOGD. In MOG, the reaction conditions allow aromatics formation. Oligomerization of isobutene from C4 olefins over zeolite catalysts has been disclosed in several US patents.
An essential feature of selective dimerization of light olefins is to prevent oligomerization, which is the successive reaction of dimerization. Selective dimerization of isobutene over trimerization and higher oligomerization is known from U.S. Pat. No. 3,325,465, DE Patent No. 3 542 171 and International Patent Application WO 01/46095. In U.S. Pat. No. 3,325,465, the use of nickel and cobalt ions in 13× zeolite is disclosed. In DE Patent No. 3 542 171, the selective dimerization of isobutene into trimethylpentene is made with bismuth or lead doped zeolite. In WO 01/46095, large pore zeolite Beta has been found to be selective for isobutene dimerization to the trimethylpentene in the presence of oxygenates. In addition, the reference teaches that dimerization can be carried out in the presence of an alcohol.
There are a few commercial dimerization processes. Institut Francais du Petrole (IFP) has developed a process for dimerization of light olefins (Dimersol). The Octol process, developed by UOP and Huls AG, produces linear octenes, which are the raw-materials of plastics softeners. Homogeneous catalysts are used in the processes.
Several processes are based on the use of ion exchange resins as dimerization catalysts. Such technical solutions are described, e.g., in U.S. Pat. Nos. 4,375,576 and 4,100,220. The known processes have many good properties but they all have the drawback of being totally dependent on oxygenate moderator, which improves the selectivity. This moderator has to be recycled and there is usually a significant amount of oxygen containing side products present in the dimerized product. When the reason to use dimerization is elimination of oxygenates from the components, these are highly undesired. Moreover, the oxygenates make hydrogenation of dimerized product more difficult.
All problems connected to oxygenates are solved if these compounds can be removed from the process altogether. Therefore, in an earlier patent application, WO 2004/080935, we have described a dimerization process, wherein an olefinic feedstock comprising C3-C5 isoolefins is contacted with a medium pore zeolite in order to dimerize the isoolefins into C6-C10 dimers. In the known process, dimerization is carried out in the presence of a heterogeneous zeolite catalyst. The catalyst can be any zeolite that is active in dimerization reactions. Such zeolites are exemplified by natural and synthetic medium pore size zeolites, such as ZSM-5, ferrierite, ZSM-22 and ZSM-23. These and similar catalysts having a pore size in the range of about 2 to 8 Å (0.2-0.8 nm), preferably 4 to 6 Å (0.4-0.6 nm), are active and selective for trimethyl olefins.